Recording of sound



IN PEN TOR.

BY w Q a ATTORNEYS:

Patented @ct. Q, 1934;

RECGli-EHNG B SOUND Application November 27, 1931, Serial No. 577,449

1 Claim.

This invention is directed to an improved method for recording sound on a film wherein the sound is recorded in the form of images of different degrees of density, one of the objects of my invention being the provision of a method whereby ground noises are less objectionable than when using the methods heretofore employed.

The present invention provides for the production or" a sound record of such a character that when this record is reproduced the ground noises are appreciably reduced and the recorded low volume sounds will be clearly audible and distortionless.

For clarity of description methods employed prior to this invention will be briefly discussed. In the well known WesternElectriclight valve system, employed extensively, (see Edward C. Wente Patent No. 1,638,555, dated August 9, i927) sound is recorded on a iilrn in the form of images of different degrees of density, a light slit or operture being imaged on the film by a suitable objective lens system. This light slit or aperture is provided by employing a metallic tape in the form of a loop, supported on insulating bridges, disposed about three mils above the face and side of an aperture provided in the pole of the light valve. The adjacent sides of this ribbon or tape act as a shutter, the space between these sides varying in width with variations in the sound curent controlled by the sound being recorded. This space between the two adjacent sides of the shutter ribbon is usually one roll in width when the apparatus is not in use and varies from less than one mil to twice this width when recording.

While satisfactory results have been obtained with this type of apparatus the realism or naturalness of the recorded sound when projected is often missing due to extraneous sounds which. are not part oi the sound to be reproduced and which are known in the art as ground noises. These ground noises may be considered as ofconstant volume and while they may not be introduced during the projection of high volume sounds such as speech and music they are decidedly noticeable during the projection of low volume sounds and quite objectionable during silent periods.

It is one of the objects of my invention to eliminate these ground noises or at least to reduce their volume to such a degree that they will no longer be objectionable during silent periods or during the reproduction of low volume sounds.

These ground noi es vary inversely to the average density or the projection positive print and if an unmodulated positive film sound track be run through a sound projector, the sound track being or" gradually varying density from minimum to maximum, the greatest amount of noise will be heard when the clear portions of the tracl; are in the sound gate, the noises gradually decreasing as the denser portion of the film comes before the gate. If, therefore, the average exposure of the negative were varied so as to be small during silent periods, that is, when no sound is being recorded, and as large as permis sible during the loudest recording passages, then the projection print from such a negative would yield more satisfactory results than it is now possible to obtain with standard me hods of record,-

It will be appreciated that if a variation in the average exposure of a negative is made by arranging the spacing between the two adjacent sides of the light valve shutter so that during silent intervals the two sides of the loop are closer together, thereby reducing the amount of light which can reach the the average transmission of the developed negative will be higher during silent intervals than during low volume sounds, so t at with a print made from such a negative there would be an inverse variation in average light transmission during silent periods, and when low volume sounds are beingrcptoduced the average transmission would be low and as a result less ground noise would be heard during silent periods. Likewise during the repro-- duction of loud passages the ground noises would belower in intensity and be less audible and not objectionable.

It has been appreciated by technicians, engaged in this art who have devised special equipment to be added to the recording system to obtain this variation in exposure of the negative. This special equipment has been in the natureof specially designed electric circuits whose function: is to narrow the space provided by the adjacent edges of the shutter ribbon during silent intervals. However, due to inherent circuit limitations there is a lag in the operation of such apparatus so that at the beginning of sudden sound impulses, if precautions are not taken to avoid it, the two sides of the ribbon may clash. To prevent this clashing excess current is supplied to the ribbon to eiiect quick separation of the twosides of the loop, which excess current, however, introduces another difiiculty in that the passage or transition from low to high sounds is inclined to be very abrupt.

My improved method overcomes the detects above mentioned in existing apparatus in a simple and satisfactory manner.

In the drawing accompanying this application I have illustrated my invention merely diagrammatically, particularly so far as the construction of the light valve is concerned inasmuch as the construction of this part of the equipment is well known.

Referring to the drawing, 1 designates a light source of substantially fixed intensity. 2 is a light valve provided with a light aperture 3. 4 is a condensing lens system intermediate the light source 1 and the light valve for concentrating the light from the light source and directing it to the aperture 3 of the light valve.

5 designates a mechanical slit images of which are to be produced on a traveling film 6, the slit 5 being focused on the film by an objective lens system '7. The whole system is in optical alignment as will be appreciated. It will be understood that the slit 5 is restricted, the showing in the drawing being much exaggerated for purposes of illustration. Actually the slit may vary in width from say of a mil to of a mil.

Adjacent the aperture 3 of the light valve 2 I provide a ribbon or shutter 8, this shutter it being understood being vibrated or actuated in the usual way by the sound impulses being recorded. During silent periods, that is, when no sound is to be recorded the ribbon or shutter 8 remains in such a position with respect to the light aperture 3 of the light valve that the aperture 3 is efiectively reduced in width, the space designated 9 which is left between the upper edge of the shutter 8 and the upper edge of the aperture 3 being only a fraction of the total width of the aperture 3. This space 9 is out of alignment with the center of the mechanical slit 5 so that the optical train of the light rays passing through this space or aperture 9 with the shutter 8 quiescent will be ofiset with respect to the optical center of the mechanical slit 5, thereby eiTecting minimum exposure of the film during silent periods. During recording periods, however, the shutter 8 will be vibrated, as will be understood, in accordance with the sound being recorded so that the space or aperture 9 will be increased in width and will be in line with the optical center of the slit 5 to produce on the film 6 images of the slit 5 with varying degrees of density.

As a result of this construction a sound track is produced on the film 6 wherein the light images of the slit 5 will be of the proper density so far as the sounds to be recorded are concerned, while that part of the film exposed during silent periods will be of reduced density. Consequently when a positive print is made from such a negative those parts of the film where the light images of the slit 5 have been produced during silent periods will be of greater density so that on reproduction a small amount of light passes through these images to the photoelectric cell upon which the light effect is negligible.

It will be appreciated from all of the foregoing, therefore, that I have provided for the recording of sound in such fashion that during silent periods the light rays from the light source 1 are passed through the light aperture, to wit, the space 9, which is so disposed with respect to the optical center of the mechanical slit 5 and the optical system 7 that only a negligible amount of light strikes the film 6. However, during recording as distinguished from silent periods the aperture 9 is in line with the optical center of the slit 5 so as to permit the necessary amount of light to pass to the film and to expose the same to the desired degree.

It will be appreciated also that my invention enables improved results to be obtained on reproduction in that ground noises are so reduced in intensity as not to be noticeable by an audience.

It will be appreciated also that low volume sounds can be reproduced clearly and satisfactorily which has been difficult heretofore owing to the ground noises.

It will be appreciated, furthermore, that my invention eliminates the difficulties experienced with prior devices wherein attempts have been made so to regulate the shutter of the light valve as to provide an aperture of minimum width during silent periods, such devices necessitating the introduction of electric circuits for preventing clashing of the edges of the ribbons constituting the shutter of the light valve, resulting in abrupt passage from minimum intensity to a much increased intensity as distinguished from the smooth action possible by my invention.

What I claim is:

Apparatus for the recording of sound, comprising in combination a light source of substantially fixed intensity, a light valve having a light aperture, a fixed mechanical slit images or" which are to be produced upon a traveling film by light rays from said light source passing through said light aperture to said slit, a sound actuated shutter for varying the opening of said aperture, said shutter being oiTset with respect to the aperture to permit light rays from said light source to pass through the said aperture during silent periods while preventing the said rays from passing to the center of said mechanical slit, actuation of said shutter by the sound being recorded during recording periods increasing the width of the aperture opening to permit the light rays from the light source to pass through said aperture and to the center of the mechanical slit and from thence to the film to produce images of the mechanical slit on the film with varying degrees of density corresponding to the sound to be recorded.

JOSEPH SHAW. 

